1. Field of the Invention
The present invention relates to optical transport systems (OTSs) and, more specifically, to improving capacity and/or cost effectiveness of an OTS during its lifespan through utilization of rate-adaptive forward error correction (FEC).
2. Description of the Related Art
In current optical transport systems (OTSs), wavelength division multiplexing (WDM) is widely used to provide adequate data-transmission capacity. The data rate of the optical signal, termed herein the “optical signal rate,” of each WDM channel is usually specified to 2.5, 10, or 40 Gb/s according to the synchronous optical network (SONET) standard and based on the projected traffic demand and optical-link conditions. Once set at the system deployment stage, the optical signal rates typically remain fixed throughout the lifespan of the OTS because changing the optical signal rates is either impossible or very expensive due to hardware and software changes.
To meet the specified quality of service (QoS) requirements, each WDM channel is generally required to guarantee a relatively low output bit error rate (BER), typically of the order of 10−16. Since the QoS requirements are expected to be met during the entire life of the OTS, a substantial initial performance margin is allocated to each WDM channel at the deployment stage in order to accommodate the subsequent loss of performance margin due to the aging of system components (e.g., optical transmitters, amplifiers, receivers, filters, and multiplexers/demultiplexers) and general degradation in the condition of the optical link. The performance margin is usually defined as the difference, in decibels (dB), between the actual signal quality (Q-factor) and the threshold Q-factor at which the system is considered to have just barely acceptable performance. The Q-factor (in dB) is related to the BER as follows:
                              B          ⁢                                          ⁢          E          ⁢                                          ⁢          R                =                              1            2                    ⁢                      erfc            ⁡                          (                                                10                                      Q                    ⁢                                                                                  ⁢                                                                  (                        dB                        )                                            /                      20                                                                                        2                                            )                                                          (        1        )            For example, for a BER of 10−3, the Q-factor is 9.8 dB. Since the typical loss of the performance margin due to aging is about 4 dB over about 20 years (a typical lifespan specified by optical network/system operators), a current OTS is often configured to have an extra performance margin of about 4 dB at the beginning of life (BOL) of the system to guarantee reliable operation over its lifespan.
Furthermore, the performance margin for a given transmission link between two optical transponders (OTs) depends on the condition of the transmission link, such as the optical signal-to-noise-ratio (OSNR) after signal transmission, chromatic dispersion (CD) and polarization-mode dispersion (PMD) characteristics, WDM crosstalk, and optical filtering penalties. The OSNR after signal transmission further depends on signal attenuation in the optical fiber, signal power, and optical amplification. It is not unusual that, at a given time, different WDM channels have different performance margins. Because, based on demand, a given WDM channel can be re-routed to establish a different transmission link between two OTs and the signal wavelength can be tuned, the performance margin for the WDM channel can vary significantly (e.g., go up and down) over time. This variation may adversely affect the overall throughput of the OTS and/or require the initial extra performance margin to be further increased above the aging-accommodation value of about 4 dB.